The present invention relates to servo actuators applicable to multi-axis drive machinery, such as robots, general purpose assembly equipment, robot hands, and other, types of multi-axis controllers. In particular, the present invention relates to a servo actuator and a position detector therefor that can detect the posture and position of a rotation axis with high accuracy.
More specifically, the present invention relates to a servo actuator including a drive circuit and to a position detector for the servo actuator. In particular, the present invention relates to a servo actuator and a position detector therefor that detect the posture and position of a rotation axis with high accuracy without being affected by switching noise in a coil current from the drive circuit.
Machinery that performs in a manner similar to human behavior using electric or magnetic operations is called a xe2x80x9crobotxe2x80x9d. It is said that the term xe2x80x9crobotxe2x80x9d derives from the Slavic word xe2x80x9cROBOTA (slave machine)xe2x80x9d. In Japan, robots have become widely used since the late 1960xe2x80x2s. Many of them are industrial robots, such as manipulators and transfer robots, designed for the purpose of automation of production operations and unmanned production operations in factories.
Stationary robots, such as arm robots, which are installed and used in particular places, operate only in a fixed and local workspace for assembly and selection of parts. In contrast, the workspace of mobile robots is not limited. Mobile robots move along a predetermined path or move freely. Therefore, mobile robots can perform predetermined or any human operations in place of human beings and can offer various services replacing human beings, dogs, and other living things. Among mobile robots, although legged mobile robots are less stable and have more difficulty in posture control and walking control compared to crawler robots and robots with tires, legged mobile robots are better in that they can climb up and down stairs and over obstacles, and can flexibly walk and run regardless whether the ground is prepared or unprepared.
Recently, research and development on legged mobile robots, such as pet robots which emulate the physical mechanisms and operations of quadrupedal walking animals such as dogs and cats, and xe2x80x9chumanoidxe2x80x9d or xe2x80x9chuman-shapedxe2x80x9d robots (humanoid robots) designed on the model of the physical mechanisms and operations of bipedal upright walking animals such as human beings, has advanced. There are increasing expectations for the practical use of legged mobile robots. For example, Sony Corporation released a bipedal walking humanoid robot xe2x80x9cSDR-3Xxe2x80x9d on Nov. 21, 2000.
Generally, a legged mobile robot of this type is equipped with many joint degrees-of-freedom, and movements of joints can be realized by actuator motors. Also, servo control is performed by calculating the rotational position and speed of each motor, thus reproducing desirable operation patterns and performing posture control.
In general, servo motors are used for realizing joint degrees-of-freedom for robots because servo motors are easy to handle, provide high torque with a compact body, and have excellent response. In particular, AC servo motors have no brush and are maintenance free, so that they can be applied to automated machinery that is desired to operate in unmanned workspaces, such as joint actuators for legged robots that can walk freely. Each AC servo motor has a permanent magnet at a rotor and coils at a stator, so that sinusoidal magnetic flux distribution and sinusoidal current cause running torque to the rotor.
Each legged mobile robot generally includes many joints. A servo motor which provides joint degrees-of-freedom is thus required to be designed and produced so as to be compact and highly efficient. For example, the specification of Japanese Patent Application No. 11-33386 (Japanese Unexamined Patent Application Publication No. 2000-299970), which has been already assigned to the applicant of the present invention, discloses a compact AC servo actuator applicable to a joint actuator of legged mobile robots. The AC servo actuator is directly connected to a gear and is of the type in which a servo control system integrated into one chip is housed in a motor unit.
For multi-axis machinery such as a legged mobile robot, rotational position of each axis must be stably detected with high accuracy in order to correctly operate the machinery by a positioning command. For example, it is required for a legged mobile robot of the bipedal upright walking-type, such as a humanoid robot, to autonomously confirm its own posture and position immediately after the power supply is switched on and to move each axis into a stable posture and position.
For this reason, the AC servo actuator which provides the rotational degrees of freedom of each joint must be provided with a higher-precision rotational position detector in order to realize such stabilization of the posture and position.
However, in the servo actuator for legged mobile robots described above, since an actuator unit includes an integrated drive circuit, there is a problem in that a position sensor signal is affected by noise generated in the drive circuit. More specifically, in the actuator unit, application of a magnetic field to a rotor causes the coil current flowing in a stator coil to be switched on and off. Switching noise in such a coil current is inevitably included in the position sensor signal.
For example, an error in the measurement accuracy of a rotation axis due to noise may result in the legged mobile robot becoming destabilized or it may even fall. If the body falls, this not only damages the robot itself but also causes accidents, such as injury to workers near the body and destruction of the object it collided with.
A conventional arrangement of the position sensor is inadequate for detecting the position of a rotation axis with high accuracy. Therefore, a sensor and a detection circuit in which noise influence is considered are needed in order to structurally separate the effects of noise.
However, a position detector in which the influence of noise is reduced needs a complicated circuit structure and high mechanical accuracy. Therefore, the detection sensor and the detection circuit become larger and expensive. Consequently, even the joint actuator itself becomes large and expensive, so that design and assembly of the overall robot becomes difficult and the cost of the apparatus is thus increased.
It is an object of the present invention to provide an excellent compact servo actuator including a drive circuit and to provide a position detector for the servo actuator.
It is another object of the present invention to provide an excellent servo actuator and a position detector therefor that detect the posture and position of a rotation axis with high accuracy without being affected by switching noise in a coil current from the drive circuit included in the servo actuator.
In view of the above objects, according to a first aspect of the present invention, a servo actuator of a type having a permanent magnet at a rotor and a coil at a stator for generating torque by a magnetic flux distribution and a current passing through the coil includes:
a casing that accommodates the rotor and the stator and supports the rotor rotatably around a predetermined rotation axis;
a drive control unit for controlling the rotation of the rotor by performing PWM (pulse width modulation) switching control on the current passing through the stator coil with a predetermined period;
a rotational position detection unit for detecting the rotational position of the rotor; and
a sampling control unit for sampling an output of the rotational position detection unit in synchronization with a switching period of the current passing through the stator coil, in the drive control unit.
Also, according to a second aspect of the present invention, a position detector for a servo actuator of a type having a permanent magnet at a rotor and a coil at a stator for generating torque by a magnetic flux distribution and a current passing through the coil includes:
a drive control unit for controlling the rotation of the rotor by performing PWM (pulse width modulation) switching control on the current passing through the stator coil with a predetermined period;
a rotational position detection unit for detecting the rotational position of the rotor; and
a sampling control unit for sampling an output of the rotational position detection unit in synchronization with a switching period of the current passing through the stator coil, in the drive control unit.
The present invention can be applied to a servo actuator having an integrated drive control circuit. The servo actuator can be applied, for example, to a joint actuator of a legged mobile robot. In this type of servo actuator, the drive control unit and the rotational position detection unit are accommodated in the casing. Therefore, the rotational position detection unit is disposed close enough to the drive control unit that the rotational position detection unit is affected by switching noise from the drive control unit.
The rotational position detection unit can be arranged with a combination of a rotor sensor magnet installed on one end face of the rotor approximately coaxially with respect to the rotation axis, the surface of the rotor sensor magnet being sinusoidally magnetized, and two rotational position sensors with a phase difference of approximate 90 degrees between each other arranged around the rotation axis at portions facing the rotor sensor magnet, the rotational position sensors detecting the magnetic flux density.
In the actuator having the integrated drive circuit, a switching current flowing in each coil within the actuator is inevitably included in a position sensor signal of the actuator. In other words, since the rotational position detection unit is disposed near the drive control unit, which functions as a switching noise source, the output signal of each rotational position sensor is superimposed with the switching current, which acts as noise.
Noise from the drive control unit includes this noise, whose fundamental component is shown by a waveform of the switching current, and other noise generated by circuit resonance due to the current variation. In this case, the noise which is superimposed on the output signal can be treated as a periodic signal which is approximately synchronized with the PWM switching period.
In the present invention, the output of the rotational position detection unit is sampled in synchronization with the switching period of the current passing through the stator coil, in the drive control unit. Accordingly, since the size of the signal whose period is identical to that of sampling becomes zero, the influence of the switching noise from the transistor can be eliminated from the sensor signal on which the noise is superimposed, even if the current switching noise is superimposed on the sensor outputs of each rotational position sensor.
A period in which the PWM switch is turned on corresponds to a transitional period of the coil current. During this period, the switching current always fluctuates, thus causing the noise to fluctuate. It is therefore relatively difficult for the noise superimposed on the sensor to be eliminated.
In contrast, during the period in which the PWM switch is turned off, the current variation is relatively stable, even in the transitional period of the coil current. The sampling control unit samples the output of the rotational position detection unit in synchronization with the period in which the drive control unit turns off the current passing through the stator coil, or in synchronization with a timing immediately before the drive control unit turns on the current passing through the stator coil. Consequently, the influence of the switching noise can be reduced.
In particular, in a dead band region allocated immediately before the PWM switch is turned on, it is ensured that the PWM switch is turned off, thus providing a stable period with minimum noise. In other words, in the dead band, the noise component contained in the sensor output is approximately constant and small.
Consequently, the sampling control unit can most efficiently reduce the influence of the switching noise for sampling the output of the rotational position detection unit in synchronization with the dead band.
Further objects, features, and advantages of the present invention will become more apparent from the following more detailed description with reference to an embodiment of the present invention and the attached drawings.